Earth X (Earth-9997)
The past history of the Deviants on Earth-9997 mostly mirrors that of their Earth-616 counterparts. However, it's been revealed that, at least on Earth-9997 the Celestials intentions for the Eternals was not exactly the beneficent intent they had thought. The Celestials method of reproduction is to impregnate the planet with a portion of their energy (or embryo) and imbed Vibranium in the planets core. Eventually after centuries of gestation the a new Celestial would be born, destroying the planet. In order to protect the Celestial growing within the planet, they would manipulate the DNA of the dominant species to act as unknowing anti-bodies. On Earth, it would be pre-historic man that would be manipulated in such a fashion. Their first attempt yeilded the Deviants, unstable creatures who were born in new and more horrifying forms within each generation, they were naturally evil. Some of the early Deviants were jump-started to the third tier of mutation, in which their identities, abilities and personality were defined by those around them. It has been identified by Kyle Richmond that these Deviants mutated into the various demons and devils that have plagued humanity, at the very least Mephisto is one of these early Deviants. In order to counteract the Deviants violent nature, the Celestials created the Eternals, but by and large, the Eternals were disinterested in protecting humanity.
The Deviants would eventually conquer the Earth and enslave the baseline human race. It has also been stated that the demons that Prince Wayfinder found (having traveled back in time to pre-historic Earth and before finding refuge in the Microverse) were early Deviants. Their enslavement of the human race did not go unnoticed, and so the Second Host of the Celestials came to Earth and wiped out their empire, which sent the Deviants underground where they never fully recovered.
In the modern age, the Deviants have interacted with humanity in much the same was as their Earth-616 counterparts. In recent history, the Deviants have played it low key on Earth during the various global, economic, and food crisis. With the exception of Ransak the Reject, who was trying to find a way to reverse his "hideous" form, and Mephisto manipulating others to create another divergent reality to escape into, most of the Deviants remained uninvolved in the current events on Earth. It was not until they were all summoned by Mephisto himself to attack Buckingham Palace and take the sword Excalibur, the only device on Earth that could still kill people following the death of Death. The Deviants and Mephisto almost succeeded if not for the intervention of King Britain, Meggan, Medusa, X-51, Black Bolt, and Ransak the Reject. During the battle Meggan died trying to keep the sword out of Mephisto's grasps. The heroes succeeded and managed to kill Mephisto. Following the defeat of their leader, the Deviants retreated. Their current whereabouts are presently unknown.